In three-phase medium voltage (MV) distribution networks, electric energy is distributed to different loads through low-voltage (LV) network feeders. The loads are connected to the feeders and could be protected using traditional LV uninterruptible power supplies (UPS). However, connecting UPS in each feeder is expensive, consumes much space, requires much maintenance, provides low overall efficiency and requires complex supervisory control. The protection of loads from upstream power quality events at medium voltage level overcomes most of the above mentioned disadvantages. Medium voltage uninterruptible power supply (MV-UPS) equips better protection mechanism for loads at MV level. Most of the existing MV-UPS use on-line UPS system technology in which AC/DC/AC conversion continuously takes place in the UPS, so that utility disturbances, e.g. failures of the power source, can be isolated from the loads.
To improve efficiency and reliability and to reduce costs, the online UPS system technology can be replaced with MV offline-UPS system technology. Offline-UPS are commonly used for power protection in industrial environments where efficiency and footprint are primary cost drivers.
In offline-UPS systems, a load, which is provided at a downstream side, is directly connected by a power bus to a power source, which is provided at an upstream side, as incoming utility supply, whereby a disconnect switch, also referred to as utility disconnect, is provided in the power bus. The power source is typically a grid supply. The offline-UPS comprise an energy storage, which is connected via a power converter to the power bus downstream of the disconnect switch.
When the offline-UPS detects a voltage disturbance, also referred to as power quality event, in the power source, the offline-UPS transfers the downstream load to backup energy storage through the power converter. Hence, the disconnect switch disconnects the load from the power source and power from the energy storage is provided via the power converter to the power bus, so that the downstream load can remain operational during the power quality event. This process of disconnecting the power source by switching off the disconnect switch and transferring the load support to the power converter with the energy storage is known as a transfer. In order to enable reliable and continuous power supply to the load also in case of power quality events, one of the most important functionalities of the MV offline-UPS is identifying power quality events.
However, in three-phase MV offline-UPS, identifying an upstream power quality event is a critical task. The utility supply voltage from the power source is sensitive to downstream load disturbances, downstream load harmonics and downstream faults due to downstream current and network impedance interaction.
Due to downstream faults, a downstream current can build up whose effect on network impedance may cause the utility voltage from the power source to violate the MV network voltage tolerance limits. This may allow the MV offline-UPS to detect a power quality event and transfer downstream load to the backup from the energy storage. Accordingly, a false detection of a power quality event may occur. This may decreases life-span of the offline-UPS due to unnecessarily increased operational time. This yields to increased efforts for maintenance and service.
Furthermore, due to high downstream currents, the MV offline-UPS may reach overload current limits, which allows it to take a decision on shedding the downstream load. Hence, there is a risk of load shedding under downstream fault detection.
MV distribution network with high downstream harmonics loads can produce significant utility voltage distortion allowing the offline-UPS to trigger a power quality event.
Further disadvantage of utility supply voltage sensitivity to downstream load disturbances, downstream load harmonics and downstream faults is inaccurate tracking of utility supply voltages.
In this context, US 2008/088183 A1 refers to a method and an apparatus for providing substantially uninterrupted power to a load. The apparatus includes a control system coupled with an electrical power storage subsystem and an electrical power generator. The control system is configured to provide a plurality of modes of operation including at least a static compensator (STATCOM) mode, an uninterruptible power supply (UPS) mode and a generator mode (gen set), and to control transitions between each of the plurality of modes. The control system is an integrated closed loop control system that includes a current control system and a voltage control system.
Furthermore, document U.S. Pat. No. 6,215,202 B1 refers to a shunt connected superconducting energy management system (SEMS). The SEMS is provided at a single switched connection between a utility grid and one or more power sensitive loads such as a semiconductor manufacturing plant having power requirements in the range on the order of 2 megawatts (MW) to 200 MW.
Still further, U.S. Pat. No. 5,172,009 A refers to a standby power supply system for supplying normal AC power to a critical load from an AC power source during normal operating conditions, and for supplying emergency AC power to the load during failure of the AC power source. During normal operation, the standby power supply system actively neutralizes undesirable harmonic components in the input current drawn by the load. The standby power supply system includes a power conversion device having a DC side coupled to a back-up power source and an AC side in parallel with the load and the AC source. A harmonic distortion sensor senses a harmonic distortion current component of a load current drawn by the load during normal operating conditions. A controller is responsive to the harmonic distortion sensor for causing the power conversion device to produce a harmonics neutralizing current to substantially neutralize the harmonic distortion current component produced by the load.